Enable storing variable-sized Vecs in UnknownArrayHandle

One of the features of UnknownArrayHandle is that it allows you to query how many Vec components each value has without resolve the type of the array. The functionality to implement this failed if you tried to store an ArrayHandle that stored Vec-like objects with Vec sizes that varied from value to value (i.e. an ArrayHandleGroupVecVariable).

Storing such an array in UnknownArrayHandle might not be the best idea, but it should probably work. This change allows you to store such an array. If you try to query the number of components, you will get 0.